Method and pad for polishing wafer

ABSTRACT

A method for polishing a wafer effectively preventing a sag in an outer peripheral portion of a wafer and a polishing pad for polishing a wafer preferably used in the method for polishing a wafer are provided. The method for polishing a wafer comprises the step of: mirror-polishing a wafer with a main surface of the wafer being in contact with a polishing pad of non-woven fabric impregnated with resin, wherein a ratio of surface roughness of the polishing pad to compressibility thereof {surface roughness Ra (μm)/compressibility (%)} is 3.8 or more.

TECHNICAL FIELD

The present invention relates to a method for polishing a wafer and apolishing pad for polishing a wafer preferably used in the method forpolishing a wafer.

BACKGROUND ART

Conventionally, a method for manufacturing a silicon wafer used as asemiconductor substrate generally includes a crystal growing process forproducing a single crystal ingot by a Czochralski (CZ) method, aFloating Zone melting (FZ) method or the like, and a wafer manufacturingprocess where this single crystal ingot is sliced and at least one mainsurface thereof is processed into a mirror-like surface. To describe theprocess more detailedly, wafer manufacturing process comprises: aslicing step of slicing the single crystal ingot to obtain a thin anddisk-shaped wafer; a chamfering step of chamfering a peripheral edgeportion of the wafer obtained through the slicing step to preventcracking and chipping of the wafer; a lapping step of flattening thiswafer; an etching step that removes machining deformation remainingbehind in the so chamfered and lapped wafer; a polishing step of makinga mirror surface of the wafer; and a cleaning step of cleaning thepolished wafer to remove abrasive slurry or dust particles depositedthereon. The main steps of the wafer manufacturing process are onlylisted above, and sometimes other steps such as a heat treating step maybe added, the above one step may be performed at multiple stages and thestep sequence may be changeable.

The polishing step, among the above steps, includes various forms ofpolishing methods. For example, a mirror-polishing method for a siliconwafer comprises various methods such as a both side polishing method topolish both surfaces of a wafer simultaneously as in the lapping step, asingle wafer polishing method to polish a single wafer vacuum-chucked ona plate one by one, and a wax-free polishing method to polish a waferheld by a backing pad and a template without using adhesive such as wax.At present, there is mainly used a polishing method using a wax mountbatch type one side polishing apparatus to polish one surfaces of pluralwafers adhered onto a plate made of such as glass or ceramics with wax.In this polishing apparatus, the plate on which the wafers are held isplaced on a turn table on which a polishing pad is adhered and with aload being applied to an upper top ring the wafer is polished whilerotating the turn table and the top ring.

Generally, there are employed as the polishing pads used in thepolishing methods polishing pads made of such as a non-woven cloth typeor a suede type. A polishing pad of a non-woven cloth type which isfabricated by impregnating a polyester felt (having a texture of arandom structure) with polyurethane, has porosity and moderateelasticity and is excellent in a polishing rate and a flatness level, sothat polishing with less of a sag is achieved. This kind of polishingpad has been widely employed for stock removal polishing of a siliconwafer.

A polishing pad of a suede type has a base material of polyester feltimpregnated with polyurethane, and a foamed layer is formed in thepolyurethane, by removing a surface portion of the foamed layer openingsbeing formed thereon (this layer is called a nap layer). This polishingpad is specifically used in final polishing, wherein abrasive slurryheld in the foamed layer acts between a work and an inner surface of thefoamed layer to thereby perform polishing of the work. Though this typeof a polishing pad has been well used in chemical mechanical polishingto obtain a damage-free surface, a peripheral sag is easy to occurduring long time polishing. In addition to the above polishing pads, apolishing pad such as a foamed urethane sheet is available.

As a method for fabricating a polishing pad such as a polishing pad of anon-woven cloth type or the like, there is exemplified a process forfabricating a polishing pad with arbitrary characteristic, wherein apolyester felt is impregnated with resin such as polyurethane and thesurface thereof is ground with a roll-shaped grindstone attached withsuper hard abrasive grains thereon (this process is called buffing). Aresin material, a content of impregnation thereof and a buffingcondition for the surface control compressibility and other propertiesof the polishing pad of this kind. At present, a polishing pad withcompressibility of the order of several % is used and it has been knownthat a polishing pad with low compressibility contributes to a reductionof the sag in an outer peripheral portion of a wafer.

It is important to keep a constant quality of each of the polishedwafers in a polishing step. For the purpose, while stabilization of apolishing pad is important, especially there become issues surfaceroughness, compressibility, a compressive elasticity modulus, andothers.

For example, if the compressibility of the polishing pad is decreased, aproblem arises that the influence of precision in a polishing apparatus(a shape of a turn table, surface deflection of the turn table,vibration in processing, etc.) cannot be absorbed to deteriorateflatness of a wafer. Conversely, if the compressibility is excessivelylarge, the amount of surface depression of the polishing pad becomeslarge to cause a sag in an outer peripheral portion of a wafer.

Also, as for the surface roughness of the polishing pad, in a case of asmoother surface thereof and a higher content of impregnated resintherein, a contact ratio between the polishing pad and an outerperipheral portion of a wafer rises to increase a sag in the outerperipheral portion. Conversely, in a case where the surface roughness ofthe polishing pad is increased by buffing it roughly, thecompressibility itself becomes larger to further increase a sag in theouter peripheral portion.

DISCLOSURE OF THE INVENTION

The present invention has been made in view of the above circumstances,and therefore an object of this invention is to provide a method forpolishing a wafer effectively preventing a sag in an outer peripheralportion of a wafer and a polishing pad for polishing a wafer preferablyused in the method for polishing a wafer.

The surface roughness and the compressibility are properties associatedwith each other, and when the surface roughness becomes coarser, thecompressibility tends to become larger. Though it is desired that inorder to reduce a sag in an outer peripheral portion of a wafer, thesurface roughness is increased to the largest value possible and thecompressibility is reduced to the lowest value possible, the propertiesconflict with each other; therefore only one of them has been controlledso far. In order to solve the above problem, the inventors haveconducted a serious study with a resultant discovery that in order tokeep the flatness of the wafer, the most suitable quality of a polishingpad can be defined by employing a ratio of the properties, and haveachieved the present invention.

That is to say, a method for polishing a wafer according to the presentinvention comprises the step of: mirror-polishing a wafer with a mainsurface of the wafer being in contact with a polishing pad of non-wovenfabric impregnated with resin, wherein a ratio of surface roughness ofthe polishing pad to compressibility thereof {surface roughness Ra(μm)/compressibility (%)} is 3.8 or more.

A polishing pad for polishing a wafer according to the present inventionis of non-woven fabric impregnated with resin, wherein a ratio ofsurface roughness of the polishing pad to compressibility thereof{surface roughness Ra (μm)/compressibility (%)} is 3.8 or more.

Since there is a tendency where the larger the ratio of the surfaceroughness to the compressibility, the better the flatness, no specificlimitation is placed on the upper limit of the ratio, while the ratio issuitably 10 or less in view of the surface roughness and thecompressibility. Especially, considering easy fabrication of thepolishing cloth, the ratio is preferably on the order of from 5 to 6.

Especially, it is preferable for the compressibility to be in the rangeof from 2% to 4.5%. If the compressibility is less than 2%, influencesof a shape of a turn table of a polishing apparatus and slight vibrationin processing cannot be absorbed to thereby generate a sag in an outerperipheral portion adversely. Moreover, if the compressibility is inexcess of 4.5%, surface depression of a polishing pad encourages a sagin an outer peripheral portion adversely.

The surface roughness is preferably controlled in the range of from 15μm to 19 μm. When a wafer is polished using a polishing pad with theabove mentioned range, a polished wafer with high quality can beobtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a relationship between {surface roughness(μ)/compressibility (%)} of a polishing pad and flatness (in a caseexcluding an outer peripheral portion of 2 mm) of a polished wafer inExperimental Example 1;

FIG. 2 is a graph showing a relationship between surface roughness (μm)of a polishing pad and flatness (in a case excluding an outer peripheralportion of 2 mm) of a polished wafer in Experimental Example 1;

FIG. 3 is a graph showing a relationship between compressibility (%) ofa polishing pad and flatness (in a case excluding an outer peripheralportion of 2 mm) of a polished wafer in Experimental Example 1; and

FIG. 4 is an explanatory sectional view for schematically showing anessential part of an embodiment of a polishing apparatus used in amethod for polishing a wafer according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will be explained below withreference to the attached drawings. This embodiment is merelyillustrative, and the present invention may be modified or changedvariously without departing from the technical idea of the presentinvention. FIG. 4 shows an embodiment of a polishing apparatus used in amethod for polishing a wafer according to the present invention and isan explanatory schematic sectional view of an essential part for showinga constructive outline of a polishing apparatus for a work equipped witha single wafer polishing head.

In FIG. 4, reference numeral 10 designates a polishing apparatus, whichis constructed as an apparatus polishing a single side of a wafer W, forexample, a semiconductor wafer such as a silicon wafer. The polishingapparatus 10 includes: a turn table 12; a polishing head 14; andabrasive slurry feed tube 16. The turn table 12 is rotated at aprescribed rotation rate by a rotary shaft 18 and a polishing pad 20 isadhered onto the top surface thereof.

Then, the wafer W is held on a wafer holding face 22 a of a waferholding plate body 22 of the polishing head 14 by vacuum chucking or thelike, and rotated by the polishing head 14 and simultaneously pressedonto the polishing pad 20 of the turn table 12 at a prescribed load.

The abrasive slurry L is fed onto the polishing pad 20 from the abrasiveslurry feed tube 16 at a prescribed flow rate, and the abrasive slurry Lis fed between the wafer W and the polishing pad 20 to thereby polish asurface of the wafer W.

The polishing head 14 is constituted of a wafer holding plate body 22with the wafer holding face 22 a and many through-holes h for vacuumchucking, and a holding back plate 24, wherein the through-holes h areconnected to a vacuum apparatus not shown via a vacuum recess 26 formedin the holding back plate 24 and via a vacuum path 28 to vacuum chuckthe wafer W on the wafer holding face 22 a by vacuum generation.

Further, there is formed a space 34 surrounded by an outer cover 30, theholding back plate 24 and an annular elastic body 32, and a compressedair feed path 36 extends into the space 34 and connected to a compressedair feeder not shown. A polishing pressure is adjustable by regulating apressure of the compressed air feeder.

Needless to say, a polishing apparatus used in a method for polishing awafer according to the present invention is not restricted to theembodiment shown in FIG. 4.

A method for polishing a wafer according to the present invention uses apolishing pad having a specific performance according to the presentinvention as the polishing pad 20, for example, in a polishing apparatus10 as shown in FIG. 4. A polishing pad used in the method of the presentinvention, namely a polishing pad according to the present inventionnecessarily has {surface roughness (μm)/compressibility (%)} of 3.8 ormore at a stage before polishing. That is to say, a quality of apolishing pad is adjusted so as to meet a prescribed condition in thepresent invention at an as-received state (at a stage when a polishingcloth is fabricated or purchased).

Here, the compressibility of a polishing pad is measured by a method inconformity with JIS L-1096. To be concrete, using a digital linear gage(for example, 1DB-112M made by Mitsutoyo Corporation), a thickness T1 isread after an elapse of 1 min after an initial load WO is applied, andthe load is increased to W1, a thickness T2 being read after an elapseof 1 min. Then, the compressibility (%) is calculated with an expressionof {(T1−T2)/T1}×100. The initial load WO is 300 g/cm² and the load W1 is1800 g/cm² for evaluation.

The surface roughness is measured on a surface of a polishing pad with aroughness measuring instrument of contact type (for example, Surfcom480A made by Tokyo Seimitsu Co., Ltd.) under prescribed conditions (ameasuring length of 12.5 mm, a measuring speed of 0.3 mm/min, a cut-offof 2.5 mm and an Ra value).

Especially, the compressibility of a polishing pad used in the method ofthe present invention, that is to say, a polishing pad according to thepresent invention, is preferably in the range of 2% to 4.5%. Therefore,the surface roughness of a polishing pad according to the presentinvention is preferably controlled to be 7.6 μm or more and morepreferably in the range of from 15 μm to 19 82 m based on an expressionof surface roughness Ra (μm)/compressibility (%)=3.8 or more, whichshows a feature of the present invention. For example, for a polishingpad with the surface roughness of 18 μm, the compressibility ispreferably on the order of from 2% to 4.7%, and for a polishing pad withthe surface roughness of 15 μm, the compressibility is preferably on theorder of from 2% to 3.9%.

Though a fabrication process for a polishing pad of the presentinvention is not different from a prior art process, it is necessary topay special attention to a relationship between the compressibility andthe surface roughness for the polishing pad of the present invention.When fabricating a polishing pad meeting the conditions required for thepolishing pad of the present invention, for example, the conditions forfabricating the polishing pad is adjusted as follows. For example, foran non-woven cloth type polishing cloth of a polyester felt impregnatedwith polyurethane, an impregnation amount of polyurethane is adjusted soas to regulate the compressibility to be on the order of 2% (slightlysmaller than 2%), followed by buffing a surface of the polishing padwith a grindstone with surface finish of about #100 to adjust itssurface roughness and thereby meet the conditions of the presentinvention.

Also, in the method according to the present invention, it is requiredto keep a ratio of {surface roughness (μm)/compressibility (%)} of thepolishing pad to be 3.8 or more even during polishing of a wafer.However, due to polishing conditions such as influences of abrasiveslurry, etc. and operating time, etc., a quality of a polishing padinevitably changes even during polishing. Therefore, in an actualpolishing operation, a ratio of {surface roughness (μm)/compressibility(%)} is adjusted to fall in the above range at least at the start stageusing the polishing pad. Also, in repeating polishing operations, aquality of the polishing pad is gradually changed, while a ratio of{surface roughness (μm)/compressibility (%)} is to be adjusted so as tobe 3.8 or more by dressing etc. of the polishing pad in the course ofthe polishing operations.

EXAMPLES

While description will be given of the present invention in a moreconcrete manner taking up examples below, it is needless to say that theexamples are shown by way of illustration only and should not beconstrued by way of limitation.

Experimental Example 1

A ratio of {surface roughness (μm)/compressibility (%)} of a polishingpad was changed to investigate a relationship between the ratio andwafer flatness after polishing. Using a flatness tester 9700 E+ made byADE Corporation, flatness was measured in terms of SFQR (Site Frontleast sQuares <site > Range: difference of undulation in each site on awafer surface) excluding an outer peripheral portion of 2 mm with a sizeof 25 mm square cell, and the max value of the cells on the wafersurface (SFQRmax) was evaluated.

With a polishing apparatus similar to the polishing apparatus shown inFIG. 4 in which there were used various polishing pads adhered thereoneach having a different ratio value of {surface roughness(μm)/compressibility (%)}, 8-inch silicon wafers subjected to etchingtreatment were polished using abrasive slurry containing colloidalsilica (at a pH value of 10.5) under polishing conditions of a polishingpressure of 300 g/cm² and a polishing time of 15 min.

FIG. 1 shows a relationship between results of the flatness (SFQR max:in a case excluding an outer peripheral portion of 2 mm) of the polishedwafer and the ratio of {surface roughness (μm)/compressibility (%)} ofthe polishing pad. As shown in FIG. 1, it was found that the ratiolargely affects the flatness and there was a good linear correlationbetween them. That is to say, it is found to be important to define aquality of the polishing pad by the ratio of {surface roughness(μm)/compressibility (%)}.

At present, there is required a quality of SFQRmax=0.13 μm or less asflatness and it is found that when defining the quality by the ratio,with the ratio of 3.8 or more, a high flatness wafer can be stablyobtained.

In addition, FIG. 2 shows a relationship between the surface roughness(μm) of the polishing pad and the flatness (excluding an outerperipheral portion of 2 mm) of the polished wafer, and FIG. 3 shows arelationship between the compressibility of the polishing pad and theflatness (in a case excluding an outer peripheral portion of 2 mm) ofthe polished wafer. As can be understood from the figures, even if thesurface roughness and the compressibility are individually controlled,their correlations with flatness are low; therefore a preferablepolishing pad cannot be defined, but by adopting the ratio of {surfaceroughness (μm)/compressibility (%)} as in the present invention, therecan be defined a polishing pad which controls the flatness easily.Especially, when the above ratio of {surface roughness(μm)/compressibility (%)} is 3.8 or more, it is preferable that therecan be easily manufactured a wafer having a flatness level to berequired hereafter.

Example 1

Using a polishing apparatus similar to the polishing apparatus shown inFIG. 4 having a polishing pad with the compressibility of 3.9% and thesurface roughness of 16.2 μm ({surface roughness (μm)/compressibility(%)} =4.15), one hundred pieces of 8-inch silicon wafers were polished.Flatness values of the polished wafers were measured under tightconditions excluding outer peripheral portions of 3 mm and 2 mm.

As a result of polishing, flatness values of all the polished waferswere SFQRmax=0.12 μm or less. Even when evaluation was performed on awafer surface excluding the outer peripheral portion of 2 mm, flatnessvalues were SFQRmax=0.12 μm or less; it is found that the wafers arestably polished to be flat up to the outer peripheral portion.

Example 2

Using a polishing apparatus similar to the polishing apparatus shown inFIG. 4 having a polishing pad with the compressibility of 4.6% and thesurface roughness of 17.8 μm ({surface roughness (μm)/compressibility(%)} =3.87), one hundred pieces of 8-inch silicon wafers were polished.Flatness values of the polished wafers were measured under tightconditions excluding outer peripheral portions of 3 mm and 2 mm.

As a result of polishing, flatness values of all the polished waferswere SFQRmax=0.13 μm or less. Even when evaluation was performed on awafer surface excluding the outer peripheral portion of 2 mm, flatnessvalues were SFQRmax=0.13 μm or less; it is found that the wafers arestably polished to be flat up to the outer peripheral portion.

Example 3

Using a polishing apparatus similar to the polishing apparatus shown inFIG. 4 having a polishing pad with the compressibility of 3.2% and thesurface roughness of 17.5 μm ({surface roughness (μm)/compressibility(%)}=5.47), one hundred pieces of 8-inch silicon wafers were polished.Flatness values of the polished wafers were measured under tightconditions excluding outer peripheral portions of 3 mm and 2 mm.

As a result of polishing, flatness values of all the polished waferswere SFQRmax=0.10 μm or less. Even when evaluation was performed on awafer surface excluding the outer peripheral portion of 2 mm, flatnessvalues were all SFQRmax=0.10 μm or less; it is found that wafers arestably polished to be flat up to the outer peripheral portion.

Comparative Example 1

Using a polishing apparatus similar to the polishing apparatus shown inFIG. 4 having a polishing pad with the compressibility of 4.6% and thesurface roughness of 14.5 μm ({surface roughness (μm)/compressibility(%)}=3.17), one hundred pieces of 8-inch silicon wafers were polished.Flatness values of the polished wafers were measured under tightconditions excluding outer peripheral portions of 3 mm and 2 mm.

As a result of polishing, flatness values of all the polished waferswere SFQRmax=0.14 μm or so on a wafer surface excluding the outerperipheral portion of 3 mm, and there were not obtained polished waferswith flatness values of 0.13 μm or less. When evaluation was performedon a wafer surface excluding the outer peripheral portion of 2 mm, theflatness values were on the order of 0.16 μm, which are inferior tothose in a case excluding the outer peripheral portion of 3 mm; it isfound that the outer peripheral portion of each wafer was sagged.

Capability of Exploitation in Industry:

As described above, according to the polishing method using thepolishing pad of the present invention, when polishing a wafer, therecan be controlled a sag on an outer peripheral portion thereof,especially a sag on an outer peripheral portion of 3 mm or less thereof,and flatness of SFQRmax=0.13 μm or less can be achieved on a wafersurface excluding an outer peripheral portion of 2 mm.

1. A method for polishing a wafer comprising the step of:mirror-polishing a wafer with a main surface of the wafer being incontact with a polishing pad of non-woven fabric impregnated with resin,wherein a ratio of surface roughness of the polishing pad tocompressibility thereof {surface roughness Ra (μm)/compressibility(%)}is 3.8 or more.
 2. The method for polishing a wafer according toclaim 1, wherein the compressibility of the polishing pad is in therange of from 2% to 4.5%.
 3. The method for polishing a wafer accordingto claim 1 wherein the surface roughness of the polishing pad is in therange from 15 μm to 19 μm.
 4. The method for polishing a wafer accordingto claim 2, wherein the surface roughness of the polishing pad is in therange of 15 μm to 19 μm.
 5. A polishing pad for polishing a wafer, ofnon-woven fabric impregnated with resin, wherein a ratio of surfaceroughness of the polishing pad to compressibility thereof {surfaceroughness Ra (μm)/compressibility (%)} is 3.8 or more.
 6. The polishingpad for polishing a wafer according to claim 5, wherein thecompressibility of the polishing pad is in the range of from 2% to 4.5%.7. The polishing pad for polishing a wafer according to claim 5, whereinthe surface roughness of the polishing pad is in the range of from 15 μmto 19 μm.
 8. The polishing pad for polishing a wafer according to claim6, wherein the surface roughness of the polishing pad is in the range offrom 15 μm to 19 μm.